Various implants have been developed to address structural failure of various parts of the spinal column. The prior art with respect to spinal column implants falls into two general categories: intervertebral disc prostheses, and rigid vertebral body prostheses.
Vertebral body prostheses have been disclosed in Pat. such as 3,426,364, 4,401,112, 4,554,914, and 4,599,086. The devices presented in those patents are intended for situations where it is necessary to remove a vertebral body. That, in turn, requires the resection of adjacent intervertebral discs. A problem common to all of such prior devices is that they adequately provide the structure of the removed vertebral body but fail to provide the flexibility of the removed intervertebral discs.
An important aspect of this invention therefore lies in providing a prosthesis for total replacement of a vertebral body and adjacent discs that may be axially expanded during implantation to fit the cavity left by the resection and, when properly in place, provides a stress environment at the prosthesis/bone interface similar to normal in vivo conditions. Specifically, normal ranges of movement are preserved, the prosthesis permitting limited longitudinal flexure, slight compression and expansion, and even a limited degree of torsional movement that at least approximates a normal range.
Briefly, the prosthesis takes the form of a pair of housings, each having end walls and a side wall defining a chamber in which a suspension plate is located. The suspension plates of the two housings are joined by an expandable connector that, in one embodiment of the invention, takes the form of an expandable bellows and, in another embodiment of the invention, comprises a threaded shaft with reverse-direction threads that extends through threaded openings in the two suspension plates. Within each chamber, the suspension plate is surrounded by an elastomeric medium. The medium provides resistance and resilience to movement of the plates within their chambers and the extent of such movement is positively limited by the dimensional differences between each plate and the chamber in which it is carried. Since the outline of each plate conforms with and is only slightly smaller than the outline of the receiving chamber, and since both are non-circular in outline, torsional movement is limited. Because each chamber has an axial dimension substantially greater than the thickness of the plate received in it, each plate may be tipped within its chamber to an extent that, when combined with a similar tipping action of the other suspension plate in its chamber, results in a degree of "bending" that approximates the action of the components of a normal spine. In general, the extent of lateral or sagittal deflection should fall within the range of 3 to 7 degrees measured from the longitudinal midline of the prosthesis, the preferred maximum deflection being about 5 degrees.
The prosthesis also includes means for setting or fixing the spacing between the housings once the prosthesis has been inserted and expanded into supportive contact with the healthy adjoining vertebrae. In the bellows-equipped embodiment, the setting means take the form of a fluid capable of hardening after being injected into the bellows to cause expansion of the prosthesis. In the embodiment in which the expansion means take the form of a threaded shaft, the setting means comprises a pair of jam nuts that may be screwed into tight frictional engagement with the inwardly (proximally) facing surfaces of the two suspension plates.